1. Field of the Invention
The present invention relates to planar antennas of the type in which radiating elements including conductor plates are provided on the surfaces of a dielectric substrate and more particularly to the construction of a two-frequency planar antenna which is operable at two different frequencies.
2. Description of the Related Art
There have been known planar antennas including a microstrip antenna and a slot antenna and such planar antennas of a two-frequency type have been proposed in the past for purposes of increasing the bandwidth.
A known type of two-frequency microstrip antenna has been so constructed that as disclosed in Japanese Patent Unexamined Publication No. 56-141605, a radiation conductor element composed of an elliptical conductor plate is arranged on one surface of a dielectric substrate whose other surface on the opposite side is covered with a grounding conductor plate, and a feeding point of the elliptical conductor plate is provided on a straight line where it is equidistant from the major and minor axes of the conductor plate. With the antenna of this construction, the elliptical radiation conductor element can be excited from the single feeding point into major-axis and minor-axis modes of different resonant frequencies which exist independently of each other thus providing a two-point resonant type frequency characteristic and allowing the operation at the two different frequencies.
On the other hand, a known slot antenna of the two-frequency type has been constructed so that as disclosed in Japanese Patent Unexamined Publication No. 58-54703, a slot is formed in a conductor deposited on one surface of a dielectric substrate whose other surface on the opposite side is provided with a pair of strip lines each crossing the slot near one of its ends and a branch filter is provided to feed power to each of the strip lines at one of two different frequencies. With this antenna, the length of the slot is selected equal to about half the wavelength at the higher one of the two frequencies, and also the slot is used as a minute slot for the lower frequency thereby allowing the antenna to operate at the two different frequencies.
Since both of these conventional antennas are of the construction in which the single radiating element is operated at the two different frequencies, the two operating frequencies cannot be selected by selecting them independently each other. Thus, in the case of the resonant microstrip antenna, it is possible to realize the selection of two frequencies in the form of values very close to each other, whereas in the case of the slot antenna, the determination of the higher one of the two frequencies naturally results in the determination of the lower frequency. Also, the same applies to the selection of gain, directivity, etc., and thus the conventional antennas have the disadvantage of extremely low degrees of freedom in the designing of antenna characteristics.